Towed seismic streamer cable assemblies typically include a plurality of spaced electrical devices selectively disposed therealong. Where the electrical devices are connected around an exterior of the towed seismic streamer cable, they are commonly referred to as wet units. In many applications, the wet units are inductively coupled to data communication lines within the seismic streamer.
One or more of the seismic streamer cable assemblies may be towed by a survey vessel. The wet units communicate with dry-end electronics disposed, for example, on the survey vessel via one or more communication channels. Communication channels between the wet units and dry-end electronics conventionally include either a single-ended or twisted-pair data communication line inductively coupled to the wet units. Electromagnetic coupling may be utilized to allow communication with the wet units without breaching the exterior sheath of the towed seismic streamer cable.
Conventionally, each of the wet units receives operational power from a battery disposed within the wet unit. The use of batteries as a primary power source in the plurality of spaced electrical devices may be required in practical applications because of low coupling coefficients between the underwater cable and the wet units. However, the use of batteries as the primary power source is frequently undesirable since the batteries may require replacement every few weeks or months. Replacing the batteries typically involves removing the wet units as the seismic cable is retrieved onto rolls on the survey vessel. The wet units are then individually serviced by opening the wet unit and replacing and/or recharging the existing batteries. This battery maintenance process may be highly inefficient and results in unwanted down time. Further, when lithium batteries are used, the cost of disposal and replacement of the batteries for a single vessel may exceed several hundred thousand dollars per year. Accordingly, conventional wet unit designs suffer from a number of problems.
A major problem associated with eliminating batteries from the wet unit devices is the low coupling coefficient between the wet units and the underwater cable. Although numerous attempts have been made to improve this coupling coefficient, these attempts have been less than satisfactory.
U.S. Pat. No. 4,912,684 to John T. Fowler describes a communication system which transmits both power and data signals along a one- or two-wire transmission line running the length of the underwater cable. The power signals may be used to charge batteries in wet units such as cable-leveling birds attached along the cable. The power and data signals are inductively coupled between the transmission line and the wet units by means of coils connected to the transmission line at specific locations along the streamer and associated coils disposed within each bird. However, due to a number of technical difficulties, a seismic streamer cable assembly which transfers operational power from the underwater cable directly to the wet units or to the wet units and in-streamer devices has not yet proven commercially practical.
For example, conventional transmission lines are typically configured as continuous, unbroken transmission lines running the length of the streamer cable which has traditionally been about 6 km or less. Transmission line losses in transmission lines of underwater streamer cables having a length longer than 6 km exacerbate the problems associated with powering the spaced electrical devices directly from the underwater streamer cable. Furthermore, data and/or power transmitted to electrical devices at the aft end of an underwater streamer cable are often severely attenuated. This problem may be particularly acute where data lines are also utilized to transmit power. It has been found that transmission line losses and noise levels in such a system often make the system commercially impractical. Thus, communication with and power delivery to aft electrical devices may be difficult, particularly for ever increasing cable lengths. Much research has been directed at solving this problem, but to date there has been little success.
One approach is to resort to heavy gauge wire and increase the power level transmitted to the cable. However, this is typically unacceptable because additional weight may be added to the underwater cable and because higher power levels may interfere with the operations of the seismic equipment, such as the underwater hydrophones.
Another shortcoming of conventional power distribution and/or data communication systems is that the inductive circuits utilized to couple between the underwater cable and the wet units are required to be precisely tuned within narrow margins to ensure adequate coupling of power and data to or from the electrical devices. If an electrical device fails, falls off, or is otherwise damaged or removed from the underwater cable, the associated coil on the transmission line may have an open secondary, detuning the tuned circuit. Often, the transmission line may be detuned to the point where reliable data and power transfer is compromised.
In typical underwater sonar cables, it is difficult to transfer power along the cable at a high frequency due to the length of the cable, amount of power required to be distributed, and the noise generated by such a transfer. Accordingly, power is typically transferred along the entire length of the cable at a low frequency. However, low frequency signals are extremely inefficient when coupled across a transformer having a low coupling coefficient. Thus, configurations which couple power from the main power line may be commercially impractical in many applications.
Another shortcoming of conventional streamer power distribution and/or data communication systems may be reliability problems due to the leakage of seawater into one or more of the sections of the streamer cable. As seawater leaks into a section of the underwater streamer cable, a low-impedance path or short circuit may be formed across the transmission line. In a continuous-wire transmission line running the length of the underwater cable, the short circuit may disable the entire transmission line. When the transmission line is disabled, sensor data cannot be collected, the electrical devices cannot be powered from the underwater cable, and depth control from the survey vessel may be precluded.
Thus, there is a need for an underwater cable power distribution and/or data communication system that overcomes these and other problems and enables highly efficient and reliable transmission of power and data between the underwater cable and the electrical devices even under demanding operational conditions.